Historically the distance between the core edge and the side seal had to be fairly large, typically up to 10 mm, in order to accommodate variation, e.g,. caused by core growth during calendering of the core prior to crimping. The process now devised uses a series of calandering steps to reduce this variation and hence allow the core edge and side seal to be much closer. With a calandering step after forming the side seal it becomes possible to make sanitary napkins having a small or even no distance between the core edge and the crimped side seal.
Absorbent products are typically made out of three layers: a liquid impervious backsheet; a liquid pervious topsheet; and an absorbent core sandwiched between the backsheet and the topsheet. When putting these three parts together during the manufacturing of absorbent products it is usual to have endless backsheet roll material and topsheet roll material guided from above and below around a discontinuous absorbent core structure.
The topsheet and backsheet are joined to each other at a certain distance to the core in order to accommodate the variation of core dimensions due to the variation introduced when making the core. The joining can be continues or discontinues. The distance also has to accommodate the alignment variations between core and the joining, equipment. The resulting product therefore necessarily has a certain distance between the edge of the core and the innermost line of the edge of the joining between the topsheet and the backsheet. In particular the usual crimp seal does not allow a tight tolerance between core edge and the crimp seal, since foreign material between topsheet and backsheet would cause the crimp seal to break or not even to form.
U.S. Pat. No. 4,592,708 discloses the problem of edge definition of the core and tolerance between core edge and side seal edge. To overcome this problem this disclosure suggests to calender the absorbent core in two steps prior to combining the topsheet and the backsheet by crimping. This US patent therefore discloses an improvement step in the direction of the present invention however without satisfying the desire to approach an almost unrecognisable distance between absorbent core edge and joining edge.
Reference relating to the same subject are U.S. Pat. No. 4,888,231 and EP-A-429 393 both disclosing particular ways of calandering absorbent cores and joining the backsheet and topsheet around them. However neither of these two disclosures provides the solution provided for by the present invention.
Therefore one objective of the present invention is to provide a process by which absorbent articles, in particular sanitary napkins, can be manufactured having a crimped side seal between the topsheet and the backsheet and having sandwiched there between an absorbent core structure while the distance between the side seal and the absorbent core is minimised to a distance of not more than 6 mm with the intention to eliminate this distance.
Another objective of the present invention is to provide the process without use of additional or new equipment relative to that which is commercially available.
Yet another objective of the present invention is to provide this process for absorbent structures having a profile, that is a varying amount of absorbent material at different parts of the absorbent structure particularly having more absorbent material in the centre of the absorbent structure. Ideally this is combined with calandering to produce a uniform density of the absorbent structure despite the profile. This is achieved by so called profiled calandering using a profiled roll calender.